Too much can be toxic, but too little will contribute to bone loss and other ailments.
Imagine a nutrient that could help prevent cancer, heart disease and tuberculosis, preserve bones and thwart autoimmune diseases such as multiple sclerosis, rheumatoid arthritis and juvenile diabetes.
Too good to be true?
But that's the potential promise for vitamin D, a nutrient whose usefulness was once thought to be limited to prevention of rickets in children and severe bone loss in adults. Known as the "sunshine" vitamin because it is naturally produced when the skin is exposed to ultraviolet light, vitamin D has been garnering increasing attention recently.
"There's a drumbeat about vitamin D that is being played very loudly," says Mary Frances Picciano of the Office of Dietary Supplements at the National Institutes of Health.
Just this month, the American Journal of Clinical Nutrition published a special supplement highlighting widespread vitamin D deficiencies "in various populations throughout the world, including 'healthy' people in developed countries where it was thought that the deficiency was obsolete."
The National Cancer Institute and the federal Office of Dietary Supplements have both convened scientists to review vitamin D data this past year. And last fall, the federal Agency for Healthcare Research and Quality issued a review of vitamin D that found it essential for bone health at all ages and for prevention of falls in the elderly.
"There are a lot of benefits to vitamin D that have surfaced in the last 20 years," notes Hector DeLuca, a University of Wisconsin biochemist who has been a pioneer in vitamin D research.
Among the more intriguing findings is a recent review of 18 studies involving nearly 60,000 people that showed those who took vitamin D supplements had a 7 percent reduction in mortality compared with those who didn't take the vitamin. Another recent study found that lung cancer patients who either got a lot of sun or had a high intake of vitamin D had three times the survival rate of their counterparts with lower vitaminD levels.
Despite these encouraging results, scientists are also quick to note that much of the vitamin D research is limited to animals, not people. "We still don't have all the information to go to the next step," Picciano says.
Simple blood test
In the meantime, an increasing number of physicians use a simple blood test to check vitamin D levels in all patients, a practice that DeLuca says "is the smart thing to do."
Testing runs about $100. Experts believe that 25 to 40 nanograms per milliliter is a reasonable target for vitamin D blood levels. But, DeLuca says, population studies suggest "that if you can get to 60 to 70 nanograms per milliliter that maybe you can address some other health problems."
Rates of colon cancer, for example, are about 50 percent lower in sunny parts of the world and in regions such as Norway where there's a high consumption of fatty fish, which are rich in vitamin D. Just 3.5 ounces of salmon provides 90 percent of the daily value of vitamin D for most adults. Sardines, tuna, eggs and liver also are rich in the vitamin, as are fortified milk and margarine.
Another option is take a dietary supplement. Based on the recent vitamin D research, DeLuca began taking 1,000 international units (IU) of vitamin D daily. He also consumes a diet that includes some vitamin D-fortified foods. He says that his total daily intake hovers around 2,000 IU daily, the level that the National Academy of Sciences sets as the tolerable upper-limit intake for adults. (Adequate daily intakes are 200 IU for those 19 to 50 years old; 400 IU for adults 51 to 70; and 600 IU for those 71 or older.)
Too much vitamin D can be toxic. "I am only confident to go to about 2,000 or 3,000 IU per day," DeLuca says. "We don't have adequate safety data to go much above that."
DeLuca, dermatologists and many scientists say that because of increased skin cancer risk from exposure to the sun, it's best to get vitamin D "from fortified foods and capsules." If you prefer the sun, however, just 20 minutes of exposure without sunscreen enables the skin to produce about 20,000 IU of vitamin D -- equivalent to drinking about 400 glasses of milk. Contrary to taking mega doses of dietary supplements, sun exposure does not appear to cause vitamin D toxicity.
You can subscribe to the free Lean Plate Club e-mail newsletter at www.leanplateclub.com. Sally Squires is a writer for the Washington Post.